Conventionally, connection between an IC chip electrode and an external lead in a semiconductor device such as ICs and LSIs is formed by a bonding wire with a diameter of about 20 to 50 μm. As a bonding wire, a fine wire made of a 4-N (99.99% by mass) pure gold alloy that is a metal generally having excellent electrical conductivity is largely used. Recently, fabricated semiconductor devices are demanded to have higher packaging density and be smaller and thinner, for example, and therefore there has been a growing trend toward finer gold alloy bonding wires with a diameter of not greater than 25 μm or even not greater than 20 μm. In addition, as cost reduction is sought, use of silver alloy bonding wires instead of gold alloy bonding wires has been studied. The diameter of such a silver alloy bonding wire is greater than the diameter of a gold alloy bonding wire and is generally around 30 μm.
Generally, bonding of a silver-gold alloy bonding wire is carried out as in the case of a gold alloy bonding wire. In other words, the method called ball bonding is employed for first bonding of a silver-gold alloy bonding wire to an electrode such as one mentioned above, and the method called wedge bonding is employed for second bonding of the silver-gold alloy bonding wire to a wire on a semiconductor wiring circuit board such as one mentioned above.
Ball bonding as the first bonding is performed by forming a free air ball (FAB) to apply arc welding heat input to the tip of a silver-gold alloy bonding wire so as to melt the tip, allowing the molten ball to solidify due to surface tension so as to form a ball called an initial ball at the tip of the silver-gold alloy bonding wire, and then applying heat to the initial ball and the electrode to a temperature ranging from 150 to 300° C., followed by pressing into adherence with ultrasonic energy applied thereto so as to form bonding.
Wedge bonding as second bonding is performed by directly heating a silver-gold alloy bonding wire to a temperature ranging from 150 to 300° C. and then, with ultrasonic energy applied thereto, pressing the silver-gold alloy bonding wire into adherence to the wiring on a wiring circuit board that has been heated to a certain temperature so as to bond the silver-gold alloy bonding wire to the wiring. Subsequently to the bonding of the silver-gold alloy bonding wire to an electrode or the wiring, the workpiece is subjected to a so-called resin molding step in which a thermosetting epoxy resin and/or a thermosetting organic silicone resin is injected onto the workpiece so as to solidify them altogether, and thus a packaged semiconductor device is obtained.
Conventionally, various silver-gold alloy bonding wires made of silver-gold alloys are developed. For example, Japanese Patent Application Publication No. 11-288962 (JP 11-288962 A) (Patent Document 1 to be listed below) has for its object to “provide an Ag alloy bonding wire that can contribute to cost reduction of semiconductor devices and is highly reliable in its bonding to chips and external leads (paragraph 0005)” and discloses in Example 20 a silver-gold alloy bonding wire that is produced by melting and casting an Ag alloy of “40.0% by weight of Au, 0.0005% by weight of Ca, and 0.0005% by weight of Ge, with the remainder being made up of Ag” and then drawing the resulting lump.
Japanese Patent Application Publication No. 2009-33127 (JP 2009-33127 A) (Patent Document 2 to be listed below) discloses that “a (silver-gold alloy) bonding wire can be produced by a method that includes after measuring weights of highly pure Au and highly pure Ag as starting materials, heating and melting them in a high vacuum or in an inert atmosphere such as nitrogen and Ar, and drawing the resulting ingot using a metal die into the diameter of the final core (paragraph 0036) . . . . (omitted) . . . , such a bonding wire with an Ag content of 55 to 90% by mass has low electrical resistance and therefore is suitable for use in a device that is required to exhibit fast response properties. In addition, with the Ag content being within this range, the Au usage can be significantly reduced and therefore the cost for Au that is available at a rising price can also be reduced (paragraph 0040).” The gold (Au) content in this disclosure is set high so as to prevent silver (Ag), which otherwise readily binds to atmospheric sulfur (S) and forms sulfides, from forming sulfides.
A resin-sealed silver-gold alloy bonding wire, however, is less resistant to thermal shock than a gold alloy bonding wire and therefore has not yet been practically used in packaged semiconductors. Especially in semiconductors subjected to high temperatures, such as semiconductors in high-intensity LEDs and automobiles, high heat load tends to be applied and therefore a silver-gold alloy bonding wire with poor thermal shock resistance has not been used. In other words, a silver-gold alloy bonding wire that is more adhesive to a molding resin such as organic silicone resins and epoxy resins than a gold alloy bonding wire has a problem that it breaks at the ball neck and the like in a first bonding and/or at the site bonded by a second bonding as the molding resin expands and contracts.
A silver-gold alloy bonding wire, as in the case of a gold alloy bonding wire, is subjected to continuous drawing and ordinary tempering heat treatment and is then wound around a spool. When wound around a spool, the silver-gold alloy bonding wire that has a low melting point and contains highly pure gold (Au) and silver (Ag) adheres to itself, which presents a problem.